Field expansion is a known method for stretching an elastomeric sleeve on site before application of the sleeve over a substrate by inserting a support core with an outside diameter larger than an inside diameter of the sleeve therethrough so that the sleeve expands in the radial direction. After the support core with the sleeve stretched thereon is positioned around the substrate, the support core is removed to allow the sleeve to shrink and tighten over the substrate so that electrical, air and/or water insulation is achieved over the substrate. The substrate can be spliced cables, terminated cables or any other form that need to be insulated or protected.
Conventional sleeves for field expansion on site typically comprise a step portion at one end thereof. The step portion provides axial positioning of the sleeve when a support core is inserted from the other end of the sleeve and allows for radial expansion of the stepped end in response to the insertion, as understood by those skilled in the art. However, the step portion decreases the structural integrity of the sleeve, and thus, in practice, stress concentration may occur at the step portion which can cause cracks in the material and lead to insulation failure. Other options for applying sleeves over substrates include pre-loaded cold shrink method and push-on method. The sleeves for pre-loaded cold shrink are pre-stretched and loaded on a core in advance of reaching the installation site. The sleeves for push-on don't need field expansion. Both the sleeves for pre-loaded cold shrink and the sleeves for push-on don't have the stepped portion, and therefore don't have the integrity problem. In some cases, the sleeves for pre-loaded cold shrink and the sleeves for push-on are desired to be expanded on site for purpose of convenient installation or some other purposes. However the configuration of some sleeves for pre-loaded cold shrink and some sleeves for push-on make it difficult or infeasible to expand such kind of sleeves on site by conventional field expansion methods.